This invention relates to a titanium (or a titanium material) having high purity, and, more particularly, a highly pure titanium useful as a material for use in forming a wiring network on the surface of a semiconductor device.
On the surface of semiconductor devices, a wiring network complexly patterned for a certain use is formed by use of a conductive metallic material. In general, to form such a wiring network, a thin film of conductive metal such as polysilicon (a-Si), Al, Au and so forth is first formed on the surface of a semiconductor device by applying a sputtering method, and thereafter the thin film is treated by etching to remove portions other than the desired wiring circuits to give a wiring network remaining on the surface.
Recently, devices have been made increasingly light, thin, short and small in size, and, as a result, an effort is made to densely form the wiring network, namely, to make the width of a circuit narrower or the thickness of a circuit thinner.
With increase in integration of semiconductor device like this, there is always caused a problem that a signal delay is caused by wiring-resistance of a wiring material used, or a problem that the material is melted by the resistance heat generated in the wiring network of an actuated device when the material comprises a low-melting material.
For these reasons, there are strong demands for wiring materials that are high-melting and low-resistant at the same time, and also do not require any modification of the process in the production of LSI, VLSI or ULSI, and Ti, comparable to Mo and Ta, has attracted notice as such material.
On the other hand, a sputtering method is chiefly employed in the step of forming a conductive metallic thin film, which is a step preliminary for the formation of wiring networks on the surface of semiconductor devices.
This is a method in which certain ion species are injected into a target comprised of a constituent material for a thin film to be formed on the surface of a semiconductor device, so that the target-constituting material may be ejected to be adhered on the surface of the semiconductor device.
In applying this sputtering method, it is necessary to prepare the sputtering target with use of the metallic material mentioned above.
Namely, a titanium material is used as the target when a wiring network comprising Ti is to be formed. Here, it is essential for the titanium material to be highly pure.
This is because, when, for example, oxygen is contained in the titanium material as an impurity even in a very small amount, the titanium material itself may become brittle causing an increase in the electric resistance of the thin film formed, whereby accidents such as melt-break of wiring networks may frequently occur; in the case of heavy metals such as Fe, Ni and Cr, they may constitute a cause of a leak at the interfacial joint between VLSI or the like and the thin film formed; or, in the case of alkali metals such as Na and K, they may be readily diffused and migrated in an insulating film provided on the VLSI or the like to deteriorate properties of the device. Also, U or Th, radiating alpha rays, may damage the device, resulting in extreme lowering of the operational reliability of the device.
At present, the following three processes are widely used as the process for producing the titanium material.
The first is a process in which a titanium compound such as TiCl.sub.4 is subjected to thermal reduction by using active metals such as Na, K and Mg, and is called the Kroll process or Hunter process. The second process is a process in which the titanium compound such as TiCl.sub.4 is subjected to thermal decomposition, and is called an iodide process. The third process is a process in which a TiO.sub.2 -NaF-KF mixture or a CaCl.sub.2 -TiO.sub.2 mixture is subjected to molten salt electrolysis.
In general, the titanium material thus produced, which usually has a spongy or acicular shape, is, for example, melted by arcs in a vacuum of 1.3.times.10.sup.-2 to 1.3.times.10.sup.-3 mbar to be formed into an ingot, which is used as a target material.
However, the titanium materials produced by the conventional processes mentioned above have a purity of only about 99% to 99.9%, which can be used as sputter target to be used in 64K bit devices, but are not suitable as wiring materials when used in 256 K bit, 1 M bit or 4M bit devices.
In other words, as mentioned above, the less impurities a titanium material has, the more desirable it is as a wiring material. For example, taking account of a wiring material for an LSI of 4M bits or more, it is found that oxygen must be not more than 200 ppm; heavy metals, not more than 30 ppm; and alkali metals, not more than 0.1 ppm. However, it is very difficult according to the above conventional industrial processes to produce a titanium having such a high purity.